The present invention relates to magnetic flowmeters. More specifically, the present invention relates to magnetic flow meters with coil ground path detection.
A magnetic flowmeter measures the volumetric flow rate of a conductive fluid by detecting the velocity of the fluid passing through a magnetic field. Magnetic flowmeter systems typically include a flowtube assembly and a transmitter assembly. The flowtube assembly is installed in a process piping line, either vertically or horizontally, and includes a pipe section, a coil section and electrodes. The coils are located on opposite sides of a cross section of the pipe. The coils, energized by a coil drive current from the transmitter, develop a magnetic field along the cross section of the pipe. Two electrodes are located across the pipe from each other along a line which is perpendicular to the magnetic field. Fluid passing through the pipe is electrically conductive. As a result of the conductor movement through the magnetic field, an electric potential or electromotive force (EMF) is induced in the fluid which can be detected across the electrodes. Operation is thus based on Faraday's law of electromagnetic induction.
The coils in the magnetic flowmeter flowtube may be compromised by process fluid leaking into the coil compartment. This can cause an electrical path between the coil and electrical ground. Electrical paths between the coil and electrical ground can also arise from other sources including age or components fatigue. The electrical ground path causes the drive signal applied to the coil to be reduced because a portion of the coil drive signal flows to electrical ground. This leads to a reduction in the applied EMF and a corresponding reduction in the output from the sense electrodes. This will lead to inaccurate flow measurements.
The loss of the coil drive signal typically cannot be detected by simply measuring the coil drive current. This is because output of the coil drive control circuitry is fixed at a set current level, regardless of any current leakage to ground.